Our previous studies identified a region on chromosome 4q that was linked to measures of insulin action. A candidate gene in this region is FABP2 which encodes the human intestinal fatty acid binding protein (IFABP). We identified a polymorphism in this gene which results in an alanine (Ala54) to threonine (Thr54) substitution at amino acid 54 of IFABP. We found a significant association between the Thr54-encoding IFABP genotype (frequency = 0.29) and increased fasting lipid oxidation rates and insulin resistance, and have further shown that recombinant Thr54 protein has a higher affinity for long- chain fatty acids as compared to recombinant Ala54 protein. We further investigated the physiologic consequences of the IFABP substitution, by analyzing fatty acid transport across permanently transfected cells expressing either Ala54 and Thr54 IFABP. We found that 3H lipid was transported at a faster rate across the Thr54-expressing cells as compared to the Ala54- expressing cells. We have also analyzed the promoters of the IFABP gene from individuals who are homzygous for the Ala54 allele and are insulin sensitive and individuals who are homzygous for the Thr54 allele and are insulin resistant. We have identified several polymorphisms and two deletions which are in 100%linkage disequilibration with the Ala to Thr substitution. To assay the functional consequences of these varied promoters, we have ligated them into luciferase reporter genes and transfected these vectors into Caco-2 cells. We have found increased transcriptional activity from mutated promoters and will be collaborating with a group in Germany to determine the role of these mutations in influencing serum triglyceride levels in subjects with Syndrome X.